<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <style>
        .demo {
            width: 300px;
            height: 300px;
            margin: auto;
            background-color: red;
            clip-path: polygon(0 100%, 50% 0, 100% 100%);

        }
    </style>
     <div class="demo"></div>
</body>

</html>